Fix nested coroutines binding issue

[dbottillo]

Fix for #128 .

When running nested coroutine bindings in parallel, an exception in one will trigger a BindCancellationException in the other, essentially leaking the exception over. Then the exception handler code is crashing because receiver.result may not be ready. A diagram explaining the issue:

[michaelbull]

Thanks so much for the hard work on this @dbottillo. The diagram was very helpful.

I have one question: my understanding is that we're testing for single-nested bindings (i.e. one inside of the other), however do we need to test for further levels of nesting to ensure that errors are only propagated upwards by a single level, and don't follow the chain upwards automatically?

e.g.

val a = coroutineBinding {
    val b1 = coroutineBinding {
        val c = coroutineBinding {
            Err("c value").bind()
         }

        assertEquals(c, Err("c value"))

        Ok("b value")
     }

    assertEquals(b1, Ok("b value"))

    val b2 = Err("a value").bind()

    Ok(Unit)
}

assertEquals(a, Err("a value"))

[dbottillo]

Thanks so much for the hard work on this @dbottillo. The diagram was very helpful.

I have one question: my understanding is that we're testing for single-nested bindings (i.e. one inside of the other), however do we need to test for further levels of nesting to ensure that errors are only propagated upwards by a single level, and don't follow the chain upwards automatically?

e.g.

val a = coroutineBinding {
    val b1 = coroutineBinding {
        val c = coroutineBinding {
            Err("c value").bind()
         }

        assertEquals(c, Err("c value"))

        Ok("b value")
     }

    assertEquals(b1, Ok("b value"))

    val b2 = Err("a value").bind()

    Ok(Unit)
}

assertEquals(a, Err("a value"))

good shout! I've added a test to cover that scenario :)

[michaelbull]

Merged in d952b52